A porous carbon material is widely used as battery material, catalyst, support for catalyst and the like, and there has been increased need for a high-functional porous carbon material serving as a gaseous adsorbent among these.
To explain the above background further specifically, next-generation energy is recently more demanded in view of environmental issues, issues on exhaustion of fossil fuels and the like. Especially, hydrogen energy is highly expected as very clean energy only generating water. For achieving utilization of hydrogen energy, each technology of hydrogen production, storage and use is required, but any of these has not yet reached practicable levels. Among these, the hydrogen storage technology is particularly lagged behind. The hydrogen storage requires various properties such as storage capacity, storage/release rate, released hydrogen purity, safety, cost and cycle characteristic, and a practicable hydrogen storage method satisfying these properties has not yet developed.
In general, hydrogen storage technology can roughly be divided into compressed hydrogen storage, liquid hydrogen storage and storage by a hydrogen storage material.
The compressed hydrogen storage is a method to fill hydrogen in a high-pressure tank in which carbon fiber is layered on the outside of liner made of resin, aluminum and the like. The energy density of the latest 70 MPa-high-pressure hydrogen gas container is only about 15% of energy density of gasoline at the most in reality. Furthermore, it is necessary to increase the thickness for ensuring pressure resistance, resulting in unavoidable increase in weight, and storage capacity of hydrogen is only approximately 3.5 to 4.5% of the weight of the tank.
On the other hand, liquid hydrogen storage attains energy density of 30% of gasoline's. However, there are problems such that the container needs to be insulated and cooled because hydrogen has very low boiling point of −253° C., and that large energy is necessary for liquefying hydrogen. Consequently, the liquid hydrogen storage cannot be excellent hydrogen storage method in view of cost and energy efficiency.
Based on these circumstances, the storage by a hydrogen storage material attracts attention, and a variety of materials, including hydrogen storage alloy, organic hydride, inorganic hydride, organic metal complex and porous carbon material, is now under development. Among the above, the porous carbon material has advantages such as a great deal of resources and lightweight, and attracts attention as a practical, strong candidate.
Typical carbon material includes graphite and activated carbon. The graphite has a hexagonal crystal structure in which graphene sheets stack. Although it has been subject to research and development for application to hydrogen storage material, the interlayer distance of the layers formed by the graphene sheets is 0.334 nm, which is too narrow as a space for storing hydrogen molecule. Then, there is reported an attempt that the interlayer of the graphite is expanded to make it porous.
For example, Patent Document 1 proposes an expanded graphite technology in which the interlayer distance is expanded by acid treatment and heating treatment to make hydrogen molecules enter the interlayer to be concentrated.
However, the interlayer of the graphene sheets is maintained by van der Waals' force, so that the expanded interlayer distance is unstable even when an appropriate interlayer distance for entering of hydrogen molecules is attained, causing problems such that due to repeated use, the interlayer distance becomes too small for hydrogen to enter, or that the interlayer distance becomes too large in contrast to obtain enrichment effect.
On the other hand, the activated carbon is a carbon material in which various carbonaceous materials as raw materials are activated by water vapor or chemicals to form large volume of pore. Since the pore of the activated carbon can be generated with progression of activation, the activated carbon is characterized by very broad pore distribution.
It has been traditionally known that the micropore of the activated carbon functions as a space for hydrogen storage and stores hydrogen. However, for the activated carbon, hydrogen storage capacity per unit weight with increase in its specific surface area, but it is difficult to increase hydrogen storage capacity per unit volume. This is because the activated carbon has broad pore distribution substantially including a lot of mesopores and macropores, and large pores unsuitable for hydrogen storage occupy a large part of the whole. Also, the production of the activated carbon is not effective in view of energy cost because two heating treatments at a temperature of approximately 800 to 1000° C. are necessary for carbonization and activation.
To overcome the above substantial problems of the activated carbon, Patent Document 2, for example, discloses a technology in which void spaces present between graphenes can be compressed to densify by high-pressure compression treatment. However, it requires high-pressure compression treatment at more than 300 MPa, and even in the activated carbon obtained by the above-mentioned densification, there are a lot of pores not involving hydrogen adsorption, so that it is far from satisfaction for hydrogen storage capacity per unit volume.
Patent Document 3 discloses a porous carbon material having phenolic resin as its raw material. According to Patent Document 3, the above porous carbon material is known to show a sharp pore distribution in which pores of 0.3 to 0.6 nm or so occupy the large part of entire pores. However, the volume of the above pores is small and still insufficient for the use as a hydrogen storage material, requiring further improvement.
Also, Patent Document 4 discloses a method for producing a porous carbon material in which a micelle of a surfactant is formed in a monomer or pre-polymer, followed by polymerization to form a micelle-containing organic polymer, and then it is subject to firing and carbonization. However, the obtained carbon material is small in subnanopore volume, and it is still insufficient for the use as a hydrogen storage material.
Namely, no porous carbon material, practically used as a hydrogen storage material, has yet been attainable, and it has been required to establish a method for producing a porous carbon material having pore volume larger than conventional porous carbon materials and showing a sharp pore distribution.
On the other hand, the greenhouse effect caused by carbon dioxide, generated as a product of combustion in large quantity at factories, automobiles and the like, is now becoming an issue. Also, one of the important resources, natural gas, contains about 5 to 10% of carbon dioxide, and is required to remove the carbon dioxide for using as a fuel. In addition, it is necessary to remove carbon dioxide discharged from a human body in a hermetic environment such as spacecraft, submarine and deep submergence vehicle.
In view of these circumstances, a technology to separate and remove carbon dioxide from a mixed gas containing carbon dioxide is required.
In general, the method for separating carbon dioxide can be divided into gas absorption method, membrane separation process and gaseous adsorption method.
In the above gas absorption method, a fluid able to dissolve a large amount of carbon dioxide is brought into contact with treatment gas to incorporate carbon dioxide into the fluid. In some cases, a physical absorption fluid such as triethylene glycol and propylene carbonate may be used, and in other cases, a chemical absorption fluid such as amine solution and potassium carbonate aqueous solution may be used. This absorption method requires regenerating procedure to separate carbon dioxide for reusing the absorption fluid. Also, a large quantity of the absorption fluid is used, so that a lot of energy is needed for heating/cooling procedures of the absorption fluid.
Also, the membrane separation process involves making only a target component permeate a polymer membrane such as polyimide having carbon dioxide selectivity for separating the same. This method is unsatisfactory in small amount of permeation because gas permeates a solid polymer membrane, as well as in expensive membrane, etc.
The above background results in attracting attention on the separation by the gaseous adsorption method. In the gaseous adsorption method, a solid adsorption material is used as a third component for separation. The adsorption material may include zeolite, activated carbon, etc. The zeolite is characterized by adsorbing a lot of carbon dioxide even at low partial pressure, but it is necessary to add a dehumidification step before the adsorption when water coexists because adsorption capability to water is notably large.
Recently, airtightness of ordinary houses is improved due to development in technologies such as building materials, designing and construction. With the developments, the concentration of carbon dioxide comes to be considered necessary to be maintained in ordinary houses in addition to the above-mentioned spacecraft, submarine, deep-sea vessel and the like. Features required in carbon dioxide removal equipments for ordinary houses may include small size/lightweight, safety and high energy efficiency, and carbon dioxide adsorption with carbon material satisfying these features is becoming the focus of attention.
As described above, the porous carbon material has a great deal of potential as a gaseous adsorbent. It is believed that particularly the carbon material having many micropores is very advantageous in adsorption of hydrogen gas, carbon dioxide, carbon monoxide, methane, ethane and other lower hydrocarbon gases, which are small in gas molecular size. However, in the adsorption of any gas, it is necessary to maintain both large pore volumes for improving adsorbed gas amount and sharp pore distribution for adsorbing a specific gas.
[Patent Document 1] The Japanese Unexamined Patent Publication 2001-026414
[Patent Document 2] The Japanese Unexamined Patent Publication 2003-038953
[Patent Document 3] The Japanese Unexamined Patent Publication H5-319813
[Patent Document 4] The Japanese Unexamined Patent Publication 2006-117523